1. Field of the Invention
The present invention relates to magnetic fluids and a process for preparing the same. Particularly, the present invention relates to a magnetic fluid composition having an improved chemical stability and the process for preparing the same. More particularly, the present invention relates to a magnetic fluid composition having an improved chemical stability and the process for preparing the same where a ferrofluid is treated with a fluorocarbon containing surface modifier. Yet more particularly, the present invention relates to a magnetic fluid composition having an improved chemical stability in acidic environments and the process for preparing the same where a ferrofluid is treated with a fluorocarbon silane surface modifier.
2. Description of the Prior Art
Magnetic fluids, sometimes referred to as "ferrofluids" or magnetic colloids, are colloidal dispersions or suspensions of finely divided magnetic or magnetizable particles ranging in size between thirty and one hundred fifty angstroms and dispersed in a carrier liquid. One of the important characteristics of magnetic fluids is their ability to be positioned and held in space by a magnetic field without the need for a container. This unique property of magnetic fluids has led to their use for a variety of applications. One such use is their use as liquid seals with low drag torque where the seals do not generate particles during operation as do conventional seals. These liquid seals are widely used in computer disc drives as exclusion seals to prevent the passage of airborne particles or gases from one side of the seal to the other. In the environmental area, environmental seals are used to prevent fugitive emissions, that is emissions of solids, liquids or gases into the atmosphere, that are harmful or potentially harmful.
Other uses of magnetic fluids are as heat transfer fluids between the voice coils and the magnets of audio speakers, as damping fluids in damping applications and as bearing lubricants in hydrodynamic bearing applications. Yet another is their use as pressure seals in devices having multiple liquid seals or stages such as a vacuum rotary feedthrough seal. Typically, this type of seal is intended to maintain a pressure differential from one side of the seal to the other while permitting a rotating shaft to project into an environment in which a pressure differential exists. Oftentimes, these vacuum rotary feedthrough seals are exposed to reactive gases such as chlorine and fluorine. These types of environments cause the magnetic fluids to deteriorate more rapidly.
The magnetic particles are generally fine particles of ferrite prepared by pulverization, precipitation, vapor deposition or other similar means. From the viewpoint of purity, particle size control and productivity, precipitation is usually the preferred means for preparing the ferrite particles. The majority of industrial applications using magnetic fluids incorporate iron oxides as magnetic particles. The most suitable iron oxides for magnetic fluid applications are ferrites such as magnetite and .gamma.-ferric oxide, which is called maghemite. Ferrites and ferric oxides offer a number of physical and chemical properties to the magnetic fluid, the most important of these being saturation magnetization, viscosity, magnetic stability, and chemical stability of the whole system. To remain in suspension, the ferrite particles require a surfactant coating, also known as a dispersant to those skilled in the art, in order to prevent the particles from coagulating or agglomerating. Fatty acids, such as oleic acid, have been used as dispersing agents to stabilize magnetic particle suspensions in some low molecular-weight non-polar hydrocarbon liquids. These low molecular-weight non-polar hydrocarbon liquids are relatively volatile solvents such as kerosene, toluene and the like. Due to their relative volatility, evaporation of these volatile hydrocarbon liquids is an important drawback as it deteriorates the function of the magnetic fluid itself. Thus to be useful, a magnetic fluid must be made with a low vapor-pressure carrier liquid and not with a low-boiling point hydrocarbon liquid.
The surfactants/dispersants have two major functions. The first is to assure a permanent distance between the magnetic particles to overcome the forces of attraction caused by Van der Waal forces and magnetic attraction, i.e. to prevent coagulation or agglomeration. The second is to provide a chemical composition on the outer surface of the magnetic particle that is compatible with the liquid carrier.
The saturation magnetization (G) of magnetic fluids is a function of the disperse phase volume of magnetic materials in the magnetic fluid. In magnetic fluids, the actual disperse phase volume is equal to the phase volume of magnetic particles plus the phase volume of the attached dispersant. The higher the magnetic particle content, the higher the saturation magnetization. The type of magnetic particles in the fluid also determines the saturation magnetization of the fluid. A set volume percent of metal particles in the fluid such as cobalt and iron generates a higher saturation magnetization than the same volume percent of ferrite. The ideal saturation magnetization for a magnetic fluid is determined by the application. For instance, saturation magnetization values for exclusion seals used in hard disk drives are typically lower than those values for vacuum seals used in the semiconductor industry.
The viscosity of the magnetic fluid is a property that is preferably controlled since it affects the suitability of magnetic fluids for particular applications. The viscosity of magnetic fluids may be predicted by principles used to describe the characteristics of an ideal colloid. According to the Einstein relationship, the viscosity of an ideal colloid is EQU (N/N.sub.0)=1+.alpha..phi.
where
N=colloid viscosity PA1 N.sub.0 =carrier liquid viscosity PA1 .alpha.=a constant; and PA1 .phi.=disperse phase volume
Gel time is a function of the life expectancy of the magnetic fluid. A magnetic fluid's gel time is dependent on various factors including temperature, viscosity, volatile components in the carrier liquid and in the dispersants, and saturation magnetization. Evaporation of the carrier liquid and oxidative degradation of the dispersant occurs when the magnetic fluid is heated. Acidic degradation of the dispersant occurs when the magnetic fluid is exposed to an acid environment. Oxidative and acidic degradation of the dispersant increases the particle-to-particle attraction within the colloid resulting in gelation of the magnetic colloid at a much more rapid rate than would occur in the absence of either oxidative or acidic degradation. The actual mechanism of acidic degradation is unknown, but it is theorized that the acid attacks the magnetic particles and dissolves the surface of the particles causing the dispersant to detach.
Most of the magnetic fluids employed today have one to three types of surfactants arranged in one, two or three layers around the magnetic particles. The surfactants for magnetic fluids are long chain molecules having a chain length of at least sixteen atoms such as carbon, or a chain of carbon and oxygen, and a functional group at one end. The chain may also contain aromatic hydrocarbons. The functional group can be cationic, anionic or nonionic in nature. The functional group is attached to the outer layer of the magnetic particles by either chemical bonding or physical force or a combination of both. The chain or tail of the surfactant provides a permanent distance between the particles and compatibility with the liquid carrier.
Various magnetic fluids and the processes for making the same have been devised in the past. The oil-based carrier liquid is generally an organic molecule, either polar or nonpolar, of various chemical compositions such as hydrocarbon (polyalpha olefins, aromatic chain structure molecules), esters (polyol esters), silicone, or fluorinated and other exotic molecules with a molecular weight range up to about eight to nine thousand. Most processes use a low boiling-point hydrocarbon solvent to peptize the ferrite particles. To evaporate the hydrocarbon solvent from the resultant oil-based magnetic fluid in these processes, all of these processes require heat treatment of the magnetic fluid at about 70.degree. C. and higher or at a lower temperature under reduced pressure. Because there are a number of factors that affect the physical and chemical properties of the magnetic fluids and that improvements in one property may adversely affect another property, it is difficult to predict the effect a change in the composition or the process will have on the overall usefulness of a magnetic fluid. It is known in the art that magnetic fluids in which one of the dispersants is a fatty acid, such as oleic, linoleic, linolenic, stearic or isostearic acid, are susceptible to oxidative degradation of the dispersant system. This results in gelation of the magnetic fluid. This becomes even more of a problem when the magnetic fluid is exposed to an acidic environment.
U.S. Pat. No. 5,676,877 (1997, Borduz et al.) teaches a composition and a process for producing a chemically stable magnetic fluid having finely divided magnetic particles covered with surfactants. A surface modifier is also employed which is added to cover thoroughly the free oxidizable exterior surface of the outer layer of the particles to assure better chemical stability of the colloidal system. The surface modifier is an alkylalkoxide silane.
U.S. Pat. No. 5,013,471 (1991, Ogawa) teaches a magnetic fluid, a method of production and a magnetic seal apparatus using the magnetic fluid. The magnetic fluid has ferromagnetic particles covered with a monomolecular adsorbed film composed of a chloro-silane type surfactant having a chain with ten to twenty-five atoms of carbon. Fluorine atoms are substituted for the hydrogen atoms of the hydrocarbon chain of the chlorosilane surfactant used in this process. According to this reference, the chlorosilane surfactant has to be large enough to disperse the particles and to assure the colloidal stability of the magnetic fluid by providing sufficient distance between the particles.
U.S. Pat. No. 5,143,637 (1992, Yokouchi et al.) teaches a magnetic fluid consisting of ferromagnetic particles dispersed in an organic solvent, a low molecular weight dispersing agent, and an additive with a carbon number between twenty-five and fifteen hundred. The low molecular weight dispersing agent is used to disperse the particles in an organic carrier. In the summary of this reference, there is a discussion about using a coupling agent, such as silane, as a dispersant. However, the coupling agent has to have a large enough molecular weight to perform as a dispersant. It should be mentioned that, in U.S. Pat. No. 5,143,637, there is no particular disclosure claim directed to using silane as an additive or even as a dispersant. The thermal stability of the fluid is increased by adding a high molecular weight additive, e.g. up to twenty thousand, such as polystyrene, polypropylene, polybutene, or polybutadiene polymers.
U.S. Pat. No. 4,554,088 (1985, Whitehead et al.) teaches use of a polymeric silane as a coupling agent. The coupling agents are a special type of surface-active chemicals that have functional groups at both ends of the long chain molecules. One end of the molecule is attached to the outer oxide layer of the magnetic particles and the other end of the molecule is attached to a specific compound of interest in those applications, such as drugs, antibodies, enzymes, etc.
None of the prior art proposes or suggests the use of low molecular weight fluorocarbon silanes as surface modifiers to cover the surface area of the magnetic particles, which is not already covered by the larger-sized surfactants, for increasing a magnetic fluid's stability in acidic environments.
Therefore, what is needed is a magnetic fluid that has a low molecular weight surface modifier covering the exposed surface area of the magnetic particles, not already covered by the larger-sized surfactants, for increasing a magnetic fluid's stability in acidic environments. What is also needed is a magnetic fluid that has a low molecular weight silane-based surface modifier covering the exposed surface area of the magnetic particles, not already covered by the larger-sized surfactants, for increasing a magnetic fluid's stability in acidic environments. What is further needed is a magnetic fluid that has a low molecular weight fluorocarbon/silane based surface modifier covering the exposed surface area of the magnetic particles, not already covered by the larger-sized surfactants, for increasing a magnetic fluid's stability in acidic environments. What is yet further needed is a fluorocarbon-based, hydrocarbon-based or ester-based magnetic fluid that has a low molecular weight fluorocarbon/silane based surface modifier covering the exposed surface area of the magnetic particles, not already covered by the larger-sized surfactants, for increasing a magnetic fluid's stability in acidic environments. Finally what is needed is a process for making a fluorocarbon-based, hydrocarbon-based or ester-based magnetic fluid that has increased stability in acidic environments.